1. Field of the Invention
The present invention relates generally to solid state devices and, more particularly, to thermoelectric devices and the fabrication of same using metal filled glass.
2. Description of the Related Art
The basic theory and operation of thermoelectric devices has been developed for many years. Modern thermoelectric devices typically include an array of semiconductor elements or thermocouples which operate by using the Peltier effect. Thermoelectric devices are essentially small heat pumps which follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other apparatus used to transfer thermal energy. A principal difference is that thermoelectric devices function with solid state electrical components (thermocouples) as compared to more traditional mechanical/fluid heating and cooling components. The efficiency of a thermoelectric device is generally limited to its associated Carnot cycle efficiency reduced by a factor which is dependent upon the thermoelectric figure of merit (ZT) of the materials used in fabrication of the thermoelectric device.
The thermoelectric figure of merit (ZT) is a dimensionless measure of the effectiveness of a thermoelectric device and is related to material properties by the following equation:
ZT=S2"sgr"T/xcexaxe2x80x83xe2x80x83(1)
where S, "sgr", xcexa, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. The Seebeck coefficient (S) is a measure of how readily the respective carriers (electrons or holes) can change energy in a temperature gradient as they move across a thermoelectric element. The thermoelectric figure of merit is related to the strength of interaction of charge carriers with the lattice structure and the available energy states associated with the respective materials.
The ZT may also be stated by the equation:                     ZT        =                                            S              2                        ⁢            T                                ρ            ⁢                          xe2x80x83                        ⁢            κ                                              (        2        )            
xcfx81=electrical resistivity
"sgr"=electrical conductivity electrical conductivity=1/electrical resistivity or
"sgr"=1/xcfx81
Today""s commercially available thermoelectric materials are generally limited to use in a temperature range between 100xc2x0 K and 1100xc2x0 K with a maximum ZT value of approximately one. The efficiency of such thermoelectric power generation devices remains relatively low at approximately five to eight percent (5-8%) energy conversion efficiency. For the temperature range of xe2x88x92100xc2x0 C. to 1000xc2x0 C., maximum ZT of conventional thermoelectric materials remains limited to values of approximately one (1), except for Texe2x80x94Agxe2x80x94Gexe2x80x94Sb alloys (TAGS) which may achieve a ZT of 1.2 to 1.4 in a very narrow temperature range. Recently developed materials such as Si80Ge20 alloys used in thermoelectric generators to power spacecraft for deep space missions have a average thermoelectric figure of merit over the temperature range of operation of approximately 0.5 from 100xc2x0 C. to 1,000xc2x0 C.
Thermoelectric cooling and temperature stabilization devices are constructed by positioning semiconductor elements made from such semiconductor alloy families as Bi2Te3, Sb2Te3 and Bi2Se3 between ceramic plates. These semiconductor elements are doped to create either an excess (n-type) or a deficiency (p-type) of electrons. Typical thermoelectric devices of this type are described in U.S. Pat. No. 4,855,810, Gelb et al. According to Gelb et al., these thermoelectric cooling devices contain semiconductor elements soldered to conductors using a solder including bismuth and tin and, in higher temperature applications, gold. One such bismuth tin solder is described in U.S. Pat. No. 3,079,455, Haba. Haba describes a solder formed of tin, antimony, and bismuth.
Thermoelectric devices built with elements composed of bismuth telluride alloy materials are used in applications where they are exposed to temperatures ranging from about xe2x88x9280xc2x0 C. to about 250xc2x0 C. The performance of such thermoelectric devices made with a tin-containing solder suffers as a result of long term exposure to wide temperature ranges. In fact, the performance of the thermoelectric devices has been found to decrease about fifteen percent or more per year. Thermoelectric devices made with tin-containing solder are not truly considered serviceable at temperatures substantially above 80xc2x0 C.
One reason for the lack of serviceability is that the standard bismuth tin solder melts at 138xc2x0 C. At temperatures above 80xc2x0 C., the tin in the solder tends to diffuse rapidly into the semiconductor elements and into the crystal lattice of the semiconductor elements, where it acts as a dopant or reacts with material of the elements. Also, the tin forms a film over the surface of the material adjacent to the soldered ends. Once created, the tin film acts as a resistor connected across the elements causing a voltage drop or a short.
Gelb et al. sought to overcome the problems of tin diffusion and resistor formation by replacing the tin-based solder with a lead-antimony solder. However, at elevated temperatures, lead also diffuses and reacts with the thermoelectric semiconductor material to form a region of poor thermoelectric performance.
To prevent diffusion of lead, tin, or other metals from the solder or the copper when used as the interconnect between the elements, the industry standard has been to employ a diffusion barrier between the elements and the solder, such as nickel layered on the elements. Such a system is shown, for example, in U.S. Pat. No. 5,429,680, Fuschetti. However, this technology is very complicated, costly, and does not completely prevent diffusion of the lead, tin or other materials. Furthermore, thermoelectric devices made from material covered at the ends with metal films provide a point of relative weakness and can become a limiting factor in the service life of the device without careful engineering and testing.
In accordance with the teachings of the present invention, a solderless thermoelectric device and method of fabrication are provided.
In one embodiment, the present invention provides a thermoelectric device having a first and a second plate, each plate having a first and second surface. A plurality of interconnects between the elements are operably coupled to the first and second plates to allow the device to be coupled to a power source. An array of thermoelectric elements, having respective first and second ends is preferably disposed between the first plate and the second plate. Metal filled glass (a specific example of metal filled glass, although not limited to, is silver-filled glass) is preferably used to respectively couple the first surface of the first plate and the first ends of the thermoelectric elements and the second ends of the thermoelectric elements to the first surface of the second plate. In this embodiment the metal filled glass replaces the solder traditionally used in this type of device.
In another embodiment, the present invention provides a thermoelectric device having at least one array of alternatively positioned n-type and p-type thermoelectric elements. Each thermoelectric element has a first end and a second end. A plurality of metal filled glass interconnects are provided to connect the first ends of adjacent n-type and p-type thermoelectric elements in series and the second ends of adjacent n-type and p-type thermoelectric elements in series and to subsequently connect the array of n-type and p-type thermoelectric elements in a serpentine manner. First and second leads are operably coupled to the thermoelectric element array and a first and a second plate are preferably operably coupled to the first and second ends of the thermoelectric elements. In this embodiment, the thermoelectric elements will not require diffusion barriers of nickel or other materials.
In yet another embodiment, the present invention provides a method of fabricating a thermoelectric device. The method includes applying metal filled glass in a desired pattern to a first surface of a first plate and to a first surface of a second plate. The method further includes positioning an array of thermoelectric elements, each thermoelectric element having a first end and a second end, on the metal filled glass pattern and then curing the metal filled glass such that the thermoelectric elements are coupled to each plate.
In another embodiment, the present invention provides a method of fabricating a thermoelectric device. The method includes applying metal filled glass to a first surface of a first plate and to at least a first surface of a second plate in a desired pattern. The first ends of adjacent n-type and p-type thermoelectric elements are positioned into the metal filled glass on the first surface of the first plate and the second ends of adjacent n-type and p-type thermoelectric elements are positioned into the metal filled glass on at least the first surface of the second plate such that the n-type and p-type thermoelectric elements are connected in a serpentine manner. The metal filled glass is then cured.
In yet another embodiment, a thermoelectric device is provided. The thermoelectric device includes a first plate and a second plate with respective interconnects formed on a first surface of the first plate and on a first surface of the second plate. A plurality of thermoelectric elements having respective first ends and second ends are provided and disposed between the first and second plates. Metal filled glass is preferably used to couple the first and second ends of each thermoelectric elements with the respective interconnects.
One technical advantage provided by the present invention is a single flow process for the fabrication of thermoelectric devices.
Another technical advantage provided by the present invention is a thermoelectric device operable at temperatures above 325xc2x0 C.
An additional technical advantage provided by the present invention is a thermoelectric fabrication method which eliminates flux and does not require a cleaning step.
Another technical advantage is the elimination of one or more of the different materials presently used for the diffusion barrier, interconnect, lead wire(s) and solder by using metal filled glass.
Another technical advantage provided by the present invention is thermoelectric device bonds which, once formed, are virtually unaffected at temperatures under 400xc2x0 C.
An additional technical advantage provided by the present invention is the ability in subsequent phases of assembly to create a high temperature, solderless bond between a thermoelectric device and an external electronic or passive device such as laser diodes, microprocessors, ceramic sub-mounts and the like with little or no adverse effect on the integrity of the thermoelectric device.
An additional technical advantage provided by the present invention is the ability to bond a thermoelectric device with other components such as laser diodes, microprocessors, thermal sinks, etc., at temperatures above 325xc2x0 C. without causing reflow in the bonding areas of the thermoelectric device.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, detailed description, and claims.